a) Field of the Invention
The present invention relates to a method for controlling a numerically-controlled (NC) electric-discharge machine which machines a workpiece with electric-discharge by forwarding the electrode in relation to the workpiece according to control information including positioning commands given to the NC unit for each block, and more particularly to a method for controlling the withdrawal of the electrode in an NC electric-discharge machine, in which the electrode is withdrawn in relation to the workpiece according to a withdrawal command supplied to the NC unit during an electric-discharge machining.
b) Description of the Prior Art
The NC electric-discharge machining is a metal-removing process in which an NC unit is used to control an electric-discharge machine by which a workpiece is machined with cumulative multiple pit marks or craters created (as the result of the workpiece material removal) by generating pulsed discharges in the spark gap between the electrode and workpiece with pulsed voltages applied between the electrode and workpiece.
In such NC electric-discharge machine, when any abnormality, typically an inter-pole short-circuit, takes place between the poles (electrode and workpiece) in the course of an electric-discharge machining, it is necessary to stop the machining and remove metal particles staying between the poles and which cause the short-circuit. The control over the withdrawal of the electrode (in an opposite direction to the forwarding of the electrode) is known as disclosed in, for example, the U.S. Pat. No. 3,975,608.
The electrode-withdrawal control of this kind is called a retracing control in which when a withdrawal command is received from the inter-pole abnormality detector, the electrode is withdrawn as retraced precisely along the forwarding route of the electrode.
This will be further explained with reference to FIG. 1 showing the prior-art electrode withdrawal control in electric-discharge machining. In FIG. 1, the reference numeral 1 denotes an electrode and 2 a workpiece. For machining the workpiece 2 into the illustrated form with its material removed by the electrode 1 having the illustrated shape, it is assumed that the positioning commands included in the control information recorded in the NC tape correspond to the points a1, a2, a3 and a4 as shown in FIG. 2. Also it is assumed that a withdrawal command is received when the electrode 1 has come to a point a5 just before the point a4. In this situation, the electrode 1 will go back along the route from the point a5 through the points a3 and a2 to the point a1 in the conventional electrode-withdrawal control. It will be obvious, however, that the electrode 1 has to go over a long distance and takes a long time for this withdrawal.
FIG. 3 also shows an electrode withdrawal in an example electric-discharge machining. In this example, the electrode 1 of the illustrated shape is used to machine the workpiece 2 into the form shown under the positioning commands in the control information recorded in the NC tape. For a machining of the workpiece of which the surface roughness is to be small, for example, in this case, it is assumed that the electrode 1 is forwarded along a route from the point b1 through the points b2, b3, b4, . . . , b(n-1) to the point bn. Namely, the electrode forwarding route has a succession of many small segments of a same direction, especially, in a section extending from the point b2 to bn. When a withdrawal command is received at the point bn, the electrode 1 will go back along the route from the point bn through the points b(n-1), . . . , b4, b3 and the point b2 to the point b1. The electrode 1 takes a long time for withdrawal, especially, from the point bn to b2.
A technique associated with this electrode-retracing control is disclosed in the Japanese Unexamined Patent Publication Nos. 51-73697 and 53-6999.
In this technique, when a withdrawal command is received, the electrode 1 is withdrawn reversely following its forwarding route. Therefore, the electrode 1 has to go back over a long distance and takes a long time for the withdrawal. Namely, the efficiency of electrode withdrawal is low and also the efficiency of metal-particle removal is low.
Note that the points designated with the designating commands, that is, each of the points a1 to a4 and b1 to bn will be referred to as "forward position" herein and that the line mutually connecting the neighboring forward positions will be called "segment" herein. Also it should be noted that the machining start-side point (block start position) of each segment will be called "beginning of segment" while the machining end-side point (block forward position) is called "end of segment".